So that's.. about 2 gallons? 1 x 50% water change per week isn't really enough to prevent ammonia buildup, so try upping that to 1 x 100%, 1 x 50% per week and see if that makes a difference.

Also, a high protein food and some live food now and then will give him more energy.

Lethargy can be one of the first signs of illness, so I'd keep an eye on that. If he doesn't improve with the water changes/good food, or seesm to be getting worse at all, I'd suspect some sort of infection.

Over-feeding will do it, too. Yeah, cut back a heap! I'd slow him down to 2 pellets 2x a day. He sounds bloated -- and no wonder! These fish will eat until they literally explode, so small meals are best.

I bet he picks right up after a week of smaller meals. If he keeps being bloated after that, get back to us!